For many years nonporous silicon dioxide (also referred to as silica) has been used as the primary dielectric to separate the wires which interconnect transistors and other devices in integrated circuits. There has been a desire in recent years to replace silica with a material having a lower dielectric constant k. A primary reason for this desire is the fact that a lower dielectric constant would, at constant geometry, reduce the capacitance between the wires and ground and between different wires. A reduction in these capacitances would result in faster charging and discharging of these wires, i.e., faster signal propagation, as well as diminished cross-talk between the signals carried on different wires.
The search for a lower k dielectric has examined a wide variety of materials. A promising approach to lowering k is to introduce porosity into silica. Because the dielectric constant of air is slightly above 1, air-filled pores will tend to lower the dielectric constant of silica. Three types of porous silica have been studied: sol-gel silica, surfactant-templated mesoporous silica, and pure silica zeolites (PSZ). Sol-gel silica has been shown to have exceptionally high porosity and thus extremely low k (e.g., k=1.2) in studies in which supercritical drying was used to generate an aerogel. However, sol-gel silica with high porosity has also been found to have low mechanical strength and low thermal conductivity. The shrinkage which sol-gel silica undergoes during drying is also a concern for dielectric applications. Another drawback of sol-gel silica is that it has a wide pore size distribution. The large pores at the upper end of this wide size distribution could result in dielectric breakdown. Furthermore, sol-gel silica is hydrophilic and tends to adsorb water. Because the dielectric constant of water is about 80–90, even minor adsorption of water could increase k significantly.
Surfactant-templated mesoporous silica has also been evaluated as a potential low-k material. It has been shown that templated mesoporous silica can achieve high porosity, and thus a low k. Such silica has also been shown to have more uniform pores than sol-gel silica, obviating concerns relating to unusually large pores. However, because it is amorphous, it could face some of the same problems as sol-gel silica with respect to mechanical strength and hydrophilicity.
A further approach to a low k dielectric material is the use of pure silica zeolites. In general a zeolite is a crystal comprising a framework of tetrahedrally-bonded atoms linked by oxygens. The tetrahedrally-bonded atoms are commonly aluminum and/or silicon, and also can be, for example, phosphorus or germanium. Zeolites tend to be microporous, i.e., to have pores of size<2 nm. Some zeolites are naturally occurring minerals, of which the earliest was discovered in the eighteenth century. In recent decades there has been a great deal of progress in the synthesis of zeolites. For a discussion of zeolites in general, please refer to the Handbook of Zeolite Science and Technology (Scott M. Auerbach et al. eds., 2003).
An important characteristic of any zeolite is its framework structure, that is to say, the spatial arrangement of the tetrahedrally-bonded (T) atoms, the oxygen atoms, and the bonds between T atoms and oxygen atoms. Zeolite framework structures are designated by three letter codes (e.g., CON, LTA, MFI) assigned by the International Zeolite Organization. Obviously, multiple different zeolites of different T atom compositions can have the same zeolite framework structure.
In understanding zeolite framework structures, it is useful to think of them in a simplified representation in which all T-O-T pairs of bonds are visualized as being straight (although in reality the angle at which the two T-O bonds meet at the oxygen can vary widely within a zeolite and between different zeolites). With the simplified representation, one can view the zeolite framework structure as being a collection of T atoms arranged in a particular way in space, interconnected by straight lines which represent the T-O-T bonds. Images of simplified representations of zeolite framework structures are available, together with much other information, at http://topaz.ethz.ch/IZA-SC/StdAtlas.htm.
Unlike sol-gel silica and mesoporous silica, pure silica zeolites would be expected to have high mechanical strength and heat conductivity due to their regular crystalline structure. They also have small pores (<1 nm) with a very narrow pore size distribution. Thus the electric breakdown problem is minimized. Although the aluminosilicate zeolites are hydrophilic, pure silica zeolites are hydrophobic, which should help reduce water adsorption. Based on these considerations, Yushan Yan at the University of California at Riverside has been working on PSZ low-k films for the last five years. The first effort was focused on films of zeolites having the MFI structure. MFI was chosen because there were many known synthesis recipes available for MFI. In addition, MFI has a reasonable level of porosity, and pure-silica MFI structured zeolites are very hydrophobic. A film was produced by in situ crystallization with good mechanical characteristics, good hydrophobicity, but with k=2.7. In later studies, Yan and colleagues have reported producing a film using zeolites of the *BEA structure, achieving k=2.3.
There is therefore a need for a low-k dielectric for integrated circuit applications based on high silica containing zeolites which have a higher porosity than MFI or *BEA.